1. Field of the Invention
The present invention relates in general to an oxygen analyzing method and a device suitable for practicing the method, and more particularly to a simple method and device for accurately determining positive and negative concentrations (for example, from -10% to +20%) of oxygen in a measurement gas which changes at random between an oxidizing atmosphere and a reducing atmosphere, wherein the positive and negative oxygen concentrations are represented by a single output signal within either a positive range or a negative range.
2. Discussion of the Prior Art
Oxygen sensors utilizing an electrochemical reaction have been used in the field of controlling various industrial furnaces as used in steelworks, and various boilers. Such sensors employ a solid electrolyte such as zirconia exhibiting oxygen-ion conductivity at an elevated temperature, and are operated according to the principle of an oxygen concentration cell, to detect the oxygen concentration (oxygen partial pressure) of exhaust gases emitted by the furnaces or boilers, or atmospheres within the furances or boilers. The combustion or burning conditions of the furnaces or boilers can be monitored or controlled based on the detected oxygen concentration.
In a commonly used oxygen analyzing device for determining the oxygen concentration of such gases or atmospheres (measurement gases), an electrochemical oxygen sensing cell is constituted by a solid electrolyte body, and a pair of porous platinum electrodes disposed on the solid electrolyte body. One of the two electrodes is used as a reference electrode exposed to a reference gas such as an ambient air having a known oxygen partial pressure The other electrode is used as a measuring electrode exposed to a measurement gas such as an exhaust gas, or an atmosphere within a furance. The oxygen partial pressure of the measurement gas is determined by detecting an electromotive force which is induced between the reference and measuring electrodes, based on a difference in oxygen partial pressure between the reference gas and the measurement gas. The electromotive force induced between the two electrodes is expressed by the well known Nernst equation, and the oxygen partial pressure of the measurement gas is readily calculated based on the the measured electromotive force, according to the Nernst equation.
The composition of the measurement gas such as exhaust gases or atmospheres within furances usually changes due to a variation in the air-fuel ratio (excess air ratio) of an air-fuel mixture. More specifically, the stoichiometric exhaust gas which is produced as a result of combustion of an air-fuel mixture having the stoichiometric air/fuel ratio (excess air ratio .mu.=1), contains substantially no free oxygen. In the case where the exhaust gas is produced in combustion of an air-rich air-fuel mixture (whose excess air ratio is higher than 1), the exhaust gas is an oxidizing atmosphere which contains free oxygen in an amount corresponding to the amount of the excess air. On the other hand, if the exhaust gas is produced as a result of combustion of a fuel-rich air-fuel mixture (whose excess air ratio is lower than 1), the exhaust gas is a reducing atmosphere which does not contain free oxygen, but contains unburned components or incombustibles.
In the case where the measurement gas to be analyzed y an electromotive oxygen sensing cell changes at random between an oxidizing atmosphere and a reducing atmosphere, an electromotive force induced according to the principle of an oxygen concentration cell is varied as a function of the excess air ratio (air/fuel ratio). This relation between the induced electromotive force and the excess air ratio is known as a .lambda. curve (lambda curve), wherein the electromotive force suddenly changes in the neighborhood of the stoichiometric air/fuel ratio (excess air ratio .mu.=1). Further, the pattern or slope of the curve of the oxidizing atmosphere is different from that of the reducing atmosphere. In the light of the above, the conventional oxygen sensor is adapted such that the oxygen concentration of the oxidizing atmosphere is obtained as an O.sub.2 signal, while the negative oxygen concentration (amount of shortage of oxygen) of the reducing atmosphere is obtained as an .alpha. signal [.alpha.=(H.sub.2 +CO)/(H.sub.2 O+CO.sub.2)]. These two different output signals associated with the oxidizing and reducing atmospheres are independently produced by respective sensing elements incorporated in a single sensor body or two separate sensor bodies.
In the above arrangement, the ranges of the output signal level obtained by the two separate sensing elements for the oxidizing and reducing atmospheres are different, and therefore suitable adjustment or compensation is required to permit measurements of positive and negative oxygen concentrations on the same calibration basis. To this end, some calculating or control circuits or devices are necessary for such adjustment, and for selective processing of the two different output signals of the oxidizing and reducing atmospheres. Accordingly, the oxygen analyzing device as a whole tends to be complicated in construction.
Recently, there has been developed an oxygen sensing element which has an electrochemical sensing cell and an electrochemical pumping cell, for handling a measurement gas whose composition changes between an oxidizing and a reducing atmosphere. The pumping cell produces an electromotive force according to the principle of an oxygen concentration cell, between its measuring electrode exposed to the measurement gas, and its reference electrode exposed to a reference gas. The pumping cell is adapted to attain an oxygen pumping action for controlling the oxygen concentration of the atmosphere adjacent to the measuring electrode of the sensing cell, so that the electromotive force to be produced by the sensing cell is equal to a predetermined reference value. In this oxygen sensing element, the positive oxygen concentration of the oxidizing atmosphere, and the negative oxygen concentration (oxygen shortage amount) of the reducing atmosphere, are evaluated by detecting an amount of electric current (pumping current) that is applied to the pumping cell to achieve the intended oxygen pumping action.
According to the above arrangement having the oxygen sensing and pumping cells, however, the required pumping current (Ip) that is applied to the pumping cell while the measurement gas is a reducing atmosphere, is considerably varied depending upon the operating temperature of the sensing element. This variation means an undesirable measuring error due to a change in the sensor temperature. Described more specifically, even if the measurement gas is a reducing atmosphere obtained in combustion of an air-fuel mixture having a constant excess air ratio, namely, even if the reducing atmosphere has the same oxygen shortage amount (negative oxygen concentration), the well known water gas reaction of CO and H.sub.2 is affected by the sensor temperature, and therefore the composition of the reducing atmosphere to be measured is varied with the sensor temperature. A difference in diffusion constant between CO and H.sub.2 will cause a fluctuation in the pumping current (Ip) that is applied to the pumping cell.